I only buy Shell or Chevron and I worked in the fuel and lubs business,
When they load gasoline (petrol) at the loading racks in the depot, they also add some additives to enhance the gasoline - in Tesco's case they are minimal and, in Shell and Chevron's, they add the most detergent (called an over-treat).
I have seen the results of engines run on supermarket fuel and other low detergent brands and you quickly get a thick carbon coating that looks like nicotine around the valves and piston rings........this is what Shell and Chevron target.
In the USA Shell and Chevron are claiming 5% fuel economy over competition that has low detergent and in fact it is much higher....(results can exceed 10%). I guess here in the UK they are chicken as to challenging others counterclaims and the ad monitoring..........
One more hidden thing too - lubricants. A change recommended by your car manufacturer as to mileage or say once a year can help fuel economy quite significantly. As the lub deteriorates (the lub oil hydrocarbon chains breaking up and their additive packages wearing out), this can mean 10% cuts in consumption.....so a regular service is recommended. We can advise here on lub as well.